This application is an improvement of my earlier United States Patents, including U.S. Pat. No. 4,183,156, entitled "Insole Construction for Articles of Footwear", issued Jan. 15, 1983, and U.S. Pat. No. 4,287,250, entitled "Elastomeric Cushioning Devices for Products and Objects," issued on Sept. 1, 1981, and U.S. Pat. No. 4,340,626, entitled "diffusion pumping Apparatus Self-Inflating Device," issued July 20, 1982.
U.S. Pat. No. '156 describes a cushioning device for articles of footwear comprising a elastomeric film envelope enclosure, preferably heat-sealed, and which is permanently inflated and pressurized during manufacture. U.S. Pat. No. '250 is more general and applies to other types of cushioning products, i.e., shock absorbers, packaging liners, helmets, door and window seals, athletic meats, mattresses, personal protective padding, etc. These earlier products utilize thermoplastic elastomeric films with the described physical properties and are inflated with novel inflatant gases, i.e. "supergases" as therein described, to achieve long-term pressurization at relatively high pressures. The method of achieving this essentially permanent inflation for the useful life of the products makes use of the novel process of diffusion pumping as described in detail in my prior U.S. Pat. No. '626.
Some form of permanent inflation and the technique therefor are important with respect to commercial acceptance of inflated product or air cushion elements to be used in footwear. For example:
(1) All valving systems leak to some degree even when new and to a much greater degree when dirty.
(2) Proper cushioning requires that the air cushion or inflated product maintain a fairly precisely controlled level of pressurization, i.e., within a few pounds of the desired pressure.
(3) The user is generally impatient and will not take the necessary time or trouble to maintain the proper inflation pressure within the device.
(4) The cost of the air cushion or the product with a valving system tends to be expensive. Not only is there the cost of the valve, but the user must be provided with a pump and a pressure gage, both of which may be costly.
(5) The air cushion or inflated device may be easily over pressurized and damaged or destroyed by the user.
(6) Improper pressurization or under pressurization may result in injury to the user.
(b 7) The pump and pressure gage may not be available to the user when needed.
(8) In cushion devices having small volumes, such as cushioning elements for footwear, the volume is so small and the pressure is so high that the process of taking a pressure reading with a typical Bourden tube pressure gage will drop the pressure between 2 and 5 pounds. Thus, the user must learn to over inflate by 2 to 5 pounds before taking a reading. This can be a tricky procedure, especially for younger children.
(9) Efforts to make a gas barrier envelope comprised of a multi-layered film sandwich comprising some sort of barrier layer within the sandwich invariably fail because of delamination adjacent to the weldments or in a region of high flexural stress.
With these devices, it is important to use diffusion pumping because to make a practical long-term pressurized cushion, it was necessary to utilize a thermoplastic elastomeric envelope film possessing certain specified physical characteristics, i.e., good processability, good heat-sealing properties, superior fatigue resistance under repeated application of comparatively high cyclical loads, as well as appropriate properties of tensile strength, puncture resistance, tear-strength, and elasticity. Because these practical considerations took precedence over the barrier properties (resistance to outward diffusion of inflation gases) of the film, it was necessary to inflate with supergas(es) and use diffusion pumping by air to help maintain the internal pressure within design limits. Good barrier materials would have been desirable for maintaining inflatant pressure, but they are necessarily crystalline in structure and thus have very poor and unacceptable physical properties, especially as regards heat-sealability, fatigue resistance and elasticity. Therefore, they could not be used for these applications. In other words, one of the considerations in the selection of barrier film materials was the fact that relatively large molecular diameter inflatant gases such as the supergases mentioned were used as the inflatant and the film materials were those which would retain the supergases but permit diffusion of smaller molecular diameter gases such as those present in air whose composition is nitrogen (78%), oxygen (20.9%), carbon dioxide (0.033%) , argon (0.934%) and the other gases (neon, helium, krypton, xenon, hydrogen, methane and nitrous oxide) which collectively make up about 30 parts per million of environmental air.
Diffusion pumping is described in my earlier U.S. Pat. '626 as follows. A pair of elastomeric, selectively permeable sheets are sealed together at desired intervals along weld lines to form one or more chambers which are later inflated with a gas, or a mixture of gases, to a prescribed pressure above atmospheric. The gas or gases selected have very low diffusion rates through the permeable sheets to the exterior of the chamber(s), the nitrogen, oxygen, and argon of the surrounding air having relatively high diffusion rates through the sheets into the chambers, producing an increase in the total pressure (potential energy level) in the chambers, resulting from diffusion pumping, which is the addition of the partial pressures of the nitrogen, oxygen, and argon of the air to the partial pressure of the gas or gases in the chambers.
Since diffusion pumping with supergas as the inflatant relies on the diffusion of the gas components of air into the envelope, there is a period of time involved before a steady state internal pressure is achieved. For example, oxygen gas diffuses into the envelope rather quickly, usually in a matter of weeks. The effect is to increase the internal pressure by about 2.5 psi. Over the next months, nitrogen gas will diffuse into the envelope and the effect is gradually to increase the pressure by an increment of about 12 psi.
There is a second effect which takes place due to the elastomeric nature of the film and that is tensile relaxation or what is sometimes called creep. The gradual increase in pressure causes about a 20% increase in the volume of the envelope over its original configuration before a steady state configuration is achieved. The net effect is that over a period of time, the internal pressure increases by about 14 psi and the volume of the envelope geometry changes by expanding. As a practical matter, these changes in geometry have been compensated for by controlled manufacturing techniques to provide an effective product. Nonetheless, the change in geometry has handicapped the design of inflated products whose geometry must be closely controlled.
Having in mind that the object was to provide an inflated product which provided a cushion feel, in addition to the other advantages mentioned in the earlier identified patents, over inflation tended to produce a hard product rather than a cushion. Under inflation to compensate for later increase in internal pressure resulted in product which would "bottom out" rather than act as a cushion. The increase in pressure over a period of months was a consideration which resulted in initially filling the envelope with a mixture of supergas and air in order to provide a product which was not over inflated, thus initially providing the desired cushion feel. This did not, however, eliminate the volume growth due to tensile relaxation. The need to mix predetermined quantities of supergas and air in order to provide the cushion feel tended to complicate the manufacturing process.
While diffusion pumping using supergases and elastomeric non-crystallographic film material has operated satisfactorily, an improved product is desirable. For example, many millions of pairs of footwear have been sold in the United States and throughout the world under the trademark "AIR SOLE" and other trademarks by Nike Shoe Company. These products of Nike Shoe Company are made in accordance with one or more of the previously identified patents and are generally regarded as premium guality footwear having the benefits of a gas filled, long service life component which offers practical advantages over competitive footwear products. Even so, there is room for improvement in the currently commercial versions of the inventions of the above patents, as will be discussed.
It is also known in the art to use certain types of plastics which are essentially impermeable to diffusion of oxygen or carbon dioxide. Typically these plastics are polycarbonate materials used in the plastic bottles of the beverage industry. The difficulty with polycarbonate and similar totally impermeable plastics is the relatively low fatigue resistance and the difficulty in forming R-F welds. In order to seal such materials, it is generally necessary to heat the facing plastics to the melting point to bring about some flow. The result is that it is difficult, if not impossible with these materials, to hold a predetermined geometry and to obtain tight and good welds by heat fusion. These materials are not polar in nature they generally cannot be R-F welded successfully.
Therefore, it is an object of this invention to provide an inflated cushioning device having longer service life at the designed internal pressure and which can be accurately controlled both in terms of steady state internal pressure and geometry.
It is a further object of this invention to match more closely the tensile relaxation properties of the enclosure film with the outward flow of gases, thereby helping to maintain more constant inflatant pressure over the service life of the product.
Another object is to slow down the inward flow of ambient air during early stages (6 to 24 months) of diffusion pumping, thereby reducing the tendency of over pressurizing certain types of the devices or bringing about gradual and undesired changes in geometry.
A further object of the invention is to use more readily available, lower weight, less expensive gases that function as the captive gas.
A further object is to permit use of selected envelope films which are superior and/or less costly for some applications.
Still another object is to provide a practical inflated cushioning device which can be pressurized with air or nitrogen, or combination thereof, and maintain inflated characteristics over its service life while exposed to the duty cycle experienced by such cushioning products.